<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title></title>
</head>
<body>
  <button id="btn">Add</button>
  <script>
  var tmpEle;

  function fn(){
    var ul = document.createElement("ul")
    for(var i=0;i<10;i++){
      var li = document.createElement("li")
      ul.appendChild(li)
    }
    tmpEle = ul
    // ul = null
  }

  document.getElementById("btn").addEventListener("click",fn)

  </script>
</body>
</html>